An evolutionary model and classification scheme for nephrite jade based on veining, fabric development, and the role of dissolution–precipitation

Although nephrite jade has been collected and treasured since the Stone Age, we lack a clear understanding of how it forms during deformation and metasomatism in shear zones. Using microstructural analysis of samples from Taiwan, California, and New Zealand, we propose a conceptual model for the evolution of nephrite jade that distinguishes four nephrite types based on mode of formation and textural characteristics: (1) primary (type 1a) or folded (type 1b) vein nephrite, (2) crenulated nephrite (type 2), (3) foliated semi-nephrite (type 3), and (4) nodular or domainal nephrite (type 4). We interpret the texture of our analysed samples to represent snapshots of a progressive textural evolution similar to that experienced by other deformed and fine-grained metamorphic rocks that develop under fluid-present, greenschist-facies conditions. Our observations suggest that types 2 and 3 nephrite can evolve from vein nephrite (type 1) by the development of crenulated and foliated metamorphic fabrics, during which the most important deformation process is dissolution–precipitation. However, development of metamorphic fabrics can be interrupted by transient brittle deformation, leading to the formation of type 4 nephrite that is characterised by nodular or angular clasts of nephrite in a nephritic matrix.

"intense shearing and mechanical breakdown" accompanied by "chemical reconstitution and growth of fresh crystals" as important processes during the formation of nephrite. Subsequent work described similar processes, suggesting that nephrite forms as "recrystallization products of thermal and cataclastic metamorphism" 24 , by metasomatism aided by tectonic movement 23 or "metamorphism accompanying shearing" 22 . Leaming (1978) summarised the conditions required for nephrite formation as "a dynamic environment best produced by tectonic activity associated with faulting" combined with "changing pressure [and] temperature" 6 . Electron microscope studies of nephrite show that crystals are elongate and possess a range of shape-and crystallographic-preferred orientations, and that textures can include tightly-packed bundles of amphibole that are seemingly randomly oriented with respect to one another 2,31 . However, our understanding of the specific deformation processes leading to the formation of nephrite is limited, in part due to the difficulty of studying the microstructure of very fine-grained monomineralic aggregates, which has resulted in several competing hypotheses regarding the dominant grain-scale mechanisms at work [8][9][10]19,20,23,24,32,33 .
In this paper, we present a conceptual model for the evolution of nephrite jade that distinguishes four nephrite types based on mode of formation and textural characteristics. The model is derived from optical, scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) analysis of 50 samples (sample catalogue  in Supplementary Item 1 and Table S1), including six representative samples that are described in detail below ( Table 1). Recognition of common microstructural characteristics in these samples, together with published descriptions of nephrite microstructure, leads us to suggest that many examples of massive nephrite may evolve from tremolite-actinolite vein networks due to the progressive development (and disruption) of metamorphic fabrics mediated by the activity of dissolution-precipitation.

Results
Below, we describe the microstructural characteristics of six representative samples, and then discuss possible evolutionary pathways that may account for the development of different types of nephrite. Table 1 includes details of the location, structural setting, and mineralogy of these six samples, and additional petrological and textural descriptions are provided in Supplementary Item 2. (1) Firstly, by direct crystallisation in veins (e.g., sample MR16; Fig. 1) that are often formed by cyclic hydrofracture 11,44 . Metasomatic reactions occurring between serpentinite and calcium-and silica-rich lithologies in serpentinite-bearing shear zones 11 , or between serpentinite blocks in a quartzofeldspatic mélange 39 , release fluid leading to repeated hydrofracturing and the formation of multi-generational vein networks 11,44 . Cross-cutting relationships indicate that vein networks often continue to form even during later stages of nephrite development. The veins are sharp-walled, monomineralic, cross-cutting (Fig. 1a), and contain elongate to fibrous acicular tremolite crystals that typically grow sub-perpendicular to vein margins in a lizardite-and chrysotile-dominated serpentinite host rock ( Fig. 1b-d; Tarling et al. 11 ). In the samples studied here, the tremolite crystals in nephrite veins are typically between 1 and 30 µm long with a width of between 0.5 and 5 µm (Fig. 1b-e). Tremolite c-axes (001) are clustered at large angles to vein margins, but show substantial spread due to "fanning" of tremolite aggregates (Fig. 1d). The a-(100) and b-axes (010) are rotated around the c-axes resulting in girdle patterns (Fig. 1d). Line profiles of cumulative  www.nature.com/scientificreports/ misorientation made through individual tremolite crystals show negligible to very low degrees of internal misorientation and a lack of low-angle or subgrain boundaries within crystals (< 2°; Fig. 1e,f). The overall frequency distribution of misorientation within all measured grains shows that a large majority of grains have < 1° of internal misorientation (Fig. 1g). (2) Secondly, by dispersed growth of tremolite due to replacement of pre-existing phases, including serpentine minerals (lizardite, chrysotile) in the serpentinite matrix (e.g., sample JC13; Fig. S1) or other minerals found in wall rocks or exotic blocks contained in serpentinite shear zones 10 . Crystal sizes are broadly similar to those found in the vein tremolite described above.
These two tremolite crystallisation and growth mechanisms commonly operate concurrently, which can lead to the complete replacement of the serpentinite matrix by aggregates of acicular tremolite, accompanied by development of tremolite vein networks.
Type 1b: folded vein nephrite with incipient crenulation cleavage. Folding during deformation in shear zones leads to the formation of an incipient crenulation cleavage defined by: (1) alignment of the limbs and axial planes of (sometimes ptygmatic) folds that develop from pre-existing primary nephrite veins (e.g. sample JC13; Fig. 2a . ESBD analysis shows that tremolite crystals in fold hinge regions meet along inter-penetrating grain boundaries between individual crystals, especially if the crystals are at high angles to one another (Fig. 2e). Additionally, newly-grown tremolite crystals that lie sub-parallel to the incipient axial planar fabric have smaller degrees of internal misorientation (< 3°) than older crystals that were deformed during the folding process (up to 9°; Fig. 2e-g).
Type 2: crenulated nephrite. In cases where deformation combined with pervasive metasomatic reactions leads to the formation of massive nephrite layers or lenses 1,6 , at least three texturally distinct varieties of nephrite (types 2-4 here) can be recognised based on our own observations and previous descriptions of nephrite texture. Some nephrites contain a well-organised spaced crenulation cleavage (Fig. 3) defined by cleavage domains that represent fold limbs composed of strongly aligned tremolite crystals, separated by microlithons that represent preserved fold hinges (e.g. OU46117 and OU65872, Fig. 3; refs 9,40 ). In one example shown here ( Fig. 3a-d) there is a smooth and progressive change in the orientation of tremolite crystals across fold hinges and limbs, and each crenulation is on the order of 50 µm wide. In the other example, the folds have a chevron geometry and there is a sharp transition in both shape and crystallographic preferred orientation of tremolite crystals across the fold hinges ( Fig. 3e-h). In both of these samples, tremolite crystals are ~ 1-50 µm long, 1-10 µm wide, contain negligible to very low degrees of internal misorientation (< 2°), and lack any subgrains (Fig. 3f,g). Type 3: foliated semi-nephrite/tremolite schist. In strongly foliated semi-nephrites, which are finegrained tremolite rocks whose grains lack the interlocking characteristic of nephrites and are texturally equivalent to many fine-grained "tremolite schists" 37 , the dominant schistosity is defined by well-aligned laths and needles of tremolite (e.g. TW04; Fig. 4a,b), which in some cases also define a macroscopic lineation. The crystallographic preferred orientation (CPO) of the main schistosity is characterised by a strong alignment of the c-axes (001) parallel to the bulk foliation plane and, if present, the lineation (Fig. 4b,c). The a-(100) and b-(010) axes define weak girdles orthogonal to the c-axes, with well-defined maxima perpendicular (a-) and parallel (b-) to the foliation plane, respectively (Fig. 4c). However, although the main schistosity is dominant, there are also domains up to 50 µm wide that contain tremolite crystals with a wider range of shape and crystallographic orientations than displayed by the main fabric (e.g. domain e in Fig. 4b,d). Crystals throughout generally show negligible to very minor degrees of internal misorientation (Fig. 4e,f), with no significant development of lowangle boundaries or subgrains (Fig. 4b).  . HCF2; Fig. 5a). Samples consist entirely of tremolite aggregates that are arranged into dis-  www.nature.com/scientificreports/ tinct microstructural domains with a range of shapes and sizes, surrounded by a fine-grained matrix of acicular tremolite (Fig. 5a,b). In some samples, domains have the shape of relict fold hinges which impart a nodular texture to the nephrite (FH in Fig. 5a). In other cases, the boundaries between domains and the surrounding matrix define angular clasts (e.g. domains c and d in Fig. 5b). Internally, individual domains comprise aggregates of fine-grained acicular to elongate tremolite with well-defined shape-and crystallographic-preferred orientations ( Fig. 5b-d). However, the SPO and CPO in different domains are rotated with respect to each other, and also have different orientations compared to the surrounding tremolite matrix (Fig. 5b-d). The matrix between domains can preserve areas with a crenulated or folded fabric (Fig. 5e-h), but in this case the crenulation is not as pervasive and consistent as in the Type 2 nephrites described above.

Discussion
We interpret the texture of our analysed nephrite samples to represent snapshots of a progressive textural evolution similar to that documented in other deformed and fine-grained metamorphic rocks that develop crenulated and foliated fabrics under fluid-present, greenschist-facies conditions ( Fig. 6; refs [45][46][47]. We suggest that nephrites can form: (1) By direct precipitation in veins (as Type 1a primary vein nephrites), (2) Due to the development and evolution of metamorphic fabrics during folding and deformation in shear zones (Types 1b, 2, 3), with dissolution-precipitation as the key process mediating fabric evolution, or, (3) By brecciation and disruption of previously-formed nephrites containing metamorphic fabrics (type 4; Fig. 6). www.nature.com/scientificreports/ In the following three paragraphs, we discuss each of these scenarios separately. The common spatiotemporal association between massive nephrite layers/lenses and tremolite-actinolite vein networks suggests that many nephrites may (at least partly) originate from veins 6,8,13,[26][27][28][29]32,39,43,[48][49][50][51] . Rapid tremolite precipitation in a dynamic shear zone environment favours growth of µm-sized laths and needles, perhaps indicating unconstrained radiating growth 52 . Several examples of nephrite in the literature contain crack-seal banding and slickenfibre-coated surfaces suggesting that they represent fragments of primary (Type 1a primary vein nephrite) or folded tremolite veins (Type 1b folded vein nephrite; Fig. 6; e.g. refs 8,[26][27][28]32,39,43 ). However, this does not preclude the possibility that nephrite types 2-4 may also develop without pre-existing veins, either from disseminated tremolite that grows at the expense of serpentine (or other phases), or from tremolite-bearing reaction rinds that are subsequently deformed. In our conceptual model, one pathway leading to the development of massive nephrite layers/lenses occurs if shear zone kinematics and other conditions (e.g. strain rate) are stable for a period of time, during which progressively folded veins and metasomatic reaction zones evolve towards a spaced crenulation cleavage (Type 2 crenulated nephrite), which may ultimately be consumed by a new penetrative planar fabric containing remnant microstructures preserved in microlithons (Type 3 foliated semi-nephrite/tremolite schist). The tremolite crystals in types 1-3 nephrite samples: (1) are often elongate to fibrous acicular crystals up to tens of microns long and a few microns wide; (2) contain negligible to very low degrees of internal misorientation, typically ≤ 2°; (3) lack significant subgrain development or other indicators of recovery and recrystallisation such as gradational internal misorientation 53 ; and (4) have grain boundaries that are often interlocking and, in some instances, interpenetrating. Additionally, folded and crenulated samples contain evidence for the growth of chemically distinct and relatively strain-free tremolite crystals that contribute to the development of an incipient axial planar fabric (Supplementary Item 3, Fig. S1, Table S2), while strongly foliated semi-nephrites contain tremolite with well-defined shape and crystallographic preferred orientations that define the main schistosity and lineation. Collectively, we interpret these observations to indicate that dissolution-precipitation is the most important longterm deformation mechanism during the formation of nephrite types 1b, 2 and 3. However, it is also evident that dissolution-precipitation is sometimes accompanied by brittle fracturing that occurs within the hinge regions of folded nephrite layers (e.g. type 1b folded nephrite vein in Fig. 2c). Our interpretation of dissolution-precipitation as the dominant deformation process during the evolution of nephrite fabrics is consistent with previous studies of deformed amphibole-bearing rocks, which report a tendency for amphibole to preferentially deform by dissolution-precipitation at temperatures of < 650 °C, sometimes accompanied by brittle fracture (e.g., refs [54][55][56][57][58]. A second and perhaps more common pathway to develop massive nephrite occurs if shear zone kinematics change, or strain rate or fluid pressure transients occur, resulting in disruption or brecciation of a previouslyformed folded, crenulated, or schistose fabric (Fig. 6). In addition to the sample described here, brecciated nephrites are reported from other localities in New Zealand 8,9 and the Apennine Mountains in Italy 59 and consist of angular clasts of nephrite surrounded by a low-porosity nephritic matrix, similar to the area shown in Fig. 5b. Individual microstructural domains in these samples have strong shape-and crystallographic-preferred orientations, which we interpret to have formed mainly by a dissolution-precipitation process as described above for nephrite types 1b, 2 and 3. However, evidence for angular domain boundaries and disorganised arrangement of shape-and crystallographic-orientations within individual domains suggests that the previously-formed www.nature.com/scientificreports/ metamorphic fabrics suffered disruption due to brittle fracturing and clast rotation (e.g. cataclasis, brecciation). Disruption and brecciation results in Type 4 domainal or nodular nephrites (Fig. 6), although microstructural evidence for previously-formed fabrics can be preserved as remnant fold hinges, partially-preserved crenulation cleavages, or discrete microstructural domains with strong SPO and CPO. Although disruption and brecciation of previously-formed fabrics is the key process involved in producing type 4 nephrites, the presence of a pure nephrite matrix in between clasts also suggests that precipitation of tremolite must rapidly seal any pore space developed by fracturing and brecciation, resulting in a low-porosity, fine-grained, well-cemented and monomineralic rock. Because nephrite is formed exclusively within or close to the margins of shear zones 1,6-10, 22 , it is likely that many nephrites experience numerous cycles of fabric formation and destruction, which could be accompanied at any stage by the formation of new vein networks. Nephrites that form in large-displacement shear zones are thus likely to experience multiple cycles of fabric evolution, which may result in complicated microstructures. The model framework presented here is broadly consistent with previous hypotheses for the formation of nephrite, which have invoked the concurrent role of metasomatism and deformation 1,[8][9][10][19][20][21][22][23][24][25][26][27][28][29][30] , and identified key shear zone processes such as dissolution 20 and brecciation 9 . However, our model builds on these previous observations by specifically identifying veining and dissolution-precipitation as playing a dominant role in the formation of many nephrites, and by proposing a conceptual framework for the progressive textural evolution of nephrite that is based on well-established metamorphic concepts (Fig. 6). This model will not provide a satisfactory explanation for the textures observed in all types of nephrite. For example, due to the challenges of performing SEM and EBSD-based analyses on very fine-grained monomineralic samples, our sample set under-represents the highest grades of nephrite which appear to consist of ultra-fine grained tremolite aggregates and an overall near-homogenous texture at the hand specimen and thin section scale 1,6,31 . Future work could involve further micro-and nano-structural analysis combined with mechanical testing of nephrites, which will help to determine how fabric and crystallography relate to material properties such as fracture toughness.

Methods
Sample preparation. Most samples were obtained as thin sections from existing collections. New samples were cut perpendicular to the foliation and parallel to the lineation (where present) and prepared either as thin sections ground to 30 µm thickness, or as briquettes with nephrites embedded in epoxy resin. In cases where no foliation or lineation was perceptible, multiple thin sections and briquettes were prepared from a range of orientations to ensure that a bias was not introduced from selecting a plane in which a lower degree of grain orientations is present (e.g., a cut coinciding with a foliation plane). Thin sections and briquettes were polished progressively with diamond paste, up to a 0.3 µm polish and finally polished with a colloidal silica in a reactive alkaline suspension (OP-S, 0.25 µm, Struer).

Microstructural observations, EBSD, and EDS analysis.
Microstructural observations were initially made using a standard research-grade polarised optical microscope. Selected samples were analysed using a Zeiss Sigma Field-Emission Gun scanning electron microscope in the Otago Micro and Nanoscale Imaging (OMNI) facility at the University of Otago. Backscatter electron (BE) and secondary electron (SE) images were acquired using a 15 keV accelerating voltage and 6.6-8 mm working distance. Electron Backscatter Diffraction (EBSD) data were acquired using an HKL Synergy Integrated EDS/EBSD system (Oxford Instruments) with an accelerating voltage of 30 kV and an aperture of 300 μm. Post-acquisition EBSD data processing was performed in Channel 5 software (Oxford Instruments) and the MTEX toolbox for MATLAB. Energy-Dispersive X-Ray Spectroscopy (EDS) measurements of chemical composition were acquired with an acceleration voltage of 15 kV, a beam current of approximately 1 nA, a live count time of 60 s, and a working distance of 8.5 mm. www.nature.com/scientificreports/